There are many types of distributed generation (DG) and energy storage devices being developed and used throughout the world. These include: fuel cells; flywheels; advanced batteries; micro-turbines; Stirling engines; wind turbines; solar cells; and double layer capacitors. Each one of these devices requires a power electronic inverter at its output to make useful AC power. Typically, this is 50 or 60 Hz single or three-phase power.
A number of techniques are described in patents and literature for connecting these devices to each other and to a utility grid. Many of these techniques involve using parallel power converters that fall into two categories, one in which devices are paralleled on the DC side of the converter and the other in which devices are paralleled on the AC side of the converter.
The concept of paralleling devices on the DC side permits the use of one large inverter, thereby reducing inverter costs. This motivation for paralleling devices on the DC side is less significant today than in the past, since the cost of controls for multiple inverter systems has decreased significantly. For a larger system, the DC-side technique uses a DC distribution system with each distributed generator supplying DC power to the DC distribution system and each load having its own inverter. In this system, a single inverter failure will cause loss of load.
Paralleling devices on the AC side is inherently more reliable, since the loads are AC. No single device failure need drop the AC power to loads as long as there is some excess capacity.
The typical method used to connect a number of power electronics units in parallel is to make one master and the rest slaves. The master is a voltage source and the slaves are current sources. This method works well if the loads are linear, have no quick surges, and draw only real power. When all of these characteristics are not present, problems can arise. These problems can be overcome to some extent through the use of high bandwidth control systems between the paralleled inverters. However, these control systems are not generally applicable for large or disperse systems. In addition, the high speed communication needed between inverters in parallel causes a single point failure issue for parallel redundant power systems and thus makes the master/slave method less reliable.
Equipment has been developed for load sharing between parallel inverters in AC power systems without the use of control circuitry connected to the inverters. Examples of such systems are described in U.S. Pat. No. 5,745,356 to Tassitino, Jr. et al. and U.S. Pat. No. 6,118,680 to Wallace et al. The information needed for load sharing is obtained from the output of each inverter in these systems. The output of each inverter is adjusted based on this information so that all of the inverters in the system equally share the load. Unfortunately, these systems are not believed to share current harmonics and transients, nor do these apparently share reactive current.
U.S. Pat. No. 6,693,409 to Lynch et al. discloses power converter control systems that are well-adapted to overcome shortcomings noted above. In a number of embodiments of the Lynch et al. patent, which are also described in detail in the Detailed Description section, the power converter control system includes an impedance current regulator (106) that includes a difference unit (102) that determines a voltage error between a feedback voltage from the output side of the power converter and a voltage signal that is a product of a reference AC voltage command signal (from source 60) and a sine waveform from a phase-locked loop (80). However, under substantially balanced voltage sags/swells during steady-state or transient operation, the voltage error that drives the impedance current regulator term in those systems can be significant and contribute excessive currents if the amplitude of the feedforward signal does not match the amplitude of the voltage feedback. This can lead to power converter control signals that are outside the operational limits of the power converters.